Pimsleur's Mandarin course teaches 'both of us' as
我们两个都 (wǒ men liăng ge dōu), but in another question I asked here I recently found that
我们俩 (wǒ men liă).
Are these always interchangeable, or are there any rules concerning when to use one over the other? Are there any more ways to say 'both of us'?